Experimental analysis of the wake of an oscillating airfoil

180 Pages·2012·7.63 MB·English

by  Dohring, Claus M.

NAVAL POSTGRADUATE SCHOOL
MONTEREY, CALIFORNIA
THESIS
EXPERIMENTAL ANALYSIS OF THE WAKE
OFAN OSCILLATING AIRFOIL
by
Claus M. Dohring
June 1996
Thesis Advisor: M. F. Platzer
Approved for public release; distribution is unlimited
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June 1996 Master's Thesis
4. TITLE AND SUBTITLE 5. FUNDING NUMBERS
EXPERIMENTAL ANALYSIS OF THE WAKE
OF AN OSCILLATING AIRFOIL
6. AUTHOR(S)
Dohring, ClausM.
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Naval Postgraduate School
Monterey, CA 93943-5000
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Theviews expressed in this thesis arethose ofthe author and do not reflect the official
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13. ABSTRACT (Maximum200words)
The wake ofan airfoil which oscillates in pure plunge mode is investigated in a
water tunnel over a wide range ofreduced frequency and amplitude. The main focus of
this study is the comparison of the experimentally determined wake geometry with
numerical results from a potential flow code. The wake vortices are visualized by two-
LDV
color dye injection and velocity profiles are measured with upstream and
downstream of the airfoil. Wake signatures are examined with regard to thrust or drag
generation. There is a good agreement between calculated and experimental data of the
vortical wavelength. At high plunge velocities both approaches show a loss of wake
symmetry and theemergence ofa dual modewakebehavior.
14. subject terms Vortical Wake Visualization 15. NUMBER OF PAGES
147
Drag-/ Thrust Signature
Comparisonwith Calculation 16. PRICE CODE
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Approved for public release; distribution is unlimited
EXPERIMENTAL ANALYSIS OF THE WAKE
OF AN OSCILLATING AIRFOIL
Claus M. Dohring
Commander, GermanNavy
Dipl. Ing., Universitat der Bundeswehr, Miinchen, 1981
Submitted in partial fulfillment ofthe
requirements forthe degree of
MASTER OF SCIENCE IN ASTRONAUTICAL ENGINEERING
fromthe
NAVAL POSTGRADUATE SCHOOL
June 1996

^
A STGRADU
MnM ATE SCHOOL
MONTEREY CA
93943-5101
ABSTRACT
The wake of an airfoil which oscillates in pure plunge mode is investigated in a
water tunnel over a wide range of reduced frequency and amplitude. The main focus of
this study is the comparison of the experimentally determined wake geometry with
numerical results from a potential flow code. The wake vortices are visualized by two-
LDV
color dye injection and velocity profiles are measured with upstream and
downstream ofthe airfoil. Wake signatures are examined with regard to thrust or drag
generation. There is a good agreement between calculated and experimental data ofthe
vortical wavelength. At high plunge velocities both approaches show a loss of wake
symmetry and the emergence ofa dual mode wake behavior.
VI
TABLE OF CONTENTS
INTRODUCTION
I. 1
EXPERIMENTAL APPARATUS
n. 3
WATER TUNNEL
A. 3
SHAKER
B. 3
C
AIRFOILS
5
LASER
D. 7
m. EXPERIMENTAL PROCEDURE 9
FLOW VELOCITY
A. 9
FREQUENCY
B. 12
AMPLITUDE
C. 12
D. DYE INJECTION 13
PHOTOGRAPHY
E. 14
VORTICAL WAVELENGTH
F. 14
LASERDOPPLERVELOCIMETRY
G. 15
MATRIX OF EXPERIMENTS
H. 18
IV. RESULTS AND DISCUSSION 25
A. WAKE OF THE FLAPPINGAIRFOIL 25
B. VORTICAL WAVELENGTH AS FUNCTION OF AMPLITUDE 27
C. VORTICAL WAVELENGTH AS FUNCTION OF FREQUENCY 29
D. DOWNSTREAM VARIATION OF THE VORTICAL WAVELENGTH.. 29
E. DRAG/THRUST SIGNATURE 32
F. COMPARISON WITH CALCULATION 32
G DUAL MODE BEHAVIOUR
38
H. VELOCITY PROFILES 40
V. CONCLUSIONS 51
APPENDIX A. WAKE PHOTOGRAPHS AND VORTICAL WAVELENGTH
PLOTS 53
APPENDIX B. TABLES 95
APPENDIX C. COMPUTED WAKES 125
LIST OF REFERENCES 133
INITIAL DISTRIBUTION LIST 135
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